It is known from German DE-AS No. 16 42 474 that it is advantageous when keeping plants and animals in an aquarium, to constantly supply carbon dioxide gas to the aquarium water. It is advantageous if the gas treatment is effected by the CO.sub.2 gas diffusing into the aquarium water via a contact surface.
It has been established that upon correct dosage of CO.sub.2 gas not only the aquarium plants thrive well, but the increase in the oxygen content of the aquarium water resulting from the increased assimilation of carbon dioxide by plants also considerably improves the living conditions for aerobic organisms. The decisive factor here is that the diffusion speed of CO.sub.2 always remains correctly adjusted.
Since larger aquariums also require a large contact surface, via which the CO.sub.2 gas can diffuse into the aquarium water in order to safeguard a sufficient diffusion speed, an attempt has already been made in German DE-AS No. 16 42 474 to achieve such an enlargement of the contact surface by means of constructive alterations to the gas treatment equipment. However, this allows dimensions to be reached which leave no alternative other than housing the contact surface either outside the aquarium or on the base of aquarium. (compare FIG.4 and FIG. 6 of German DE-AS No. 16 42 474).